Although there has been spectacular recent progress in the field, the basic scheme of visual transduction in invertebrate photoreceptors remains unsolved. It seems certain now that phosphoinositide metabolism lies at th heart of invertebrate transduction. However, new studies show that intracellular cyclic-GMP regulation must be a key element in excitation of invertebrate photoreceptors. These findings dramatically illustrate that both second messenger systems must operate together within the same cell to yield visual excitation. Furthermore, there is evidence that both pathways are involved in the physiologically tractable photoreceptors of Limulus as well as in the photoreceptors of Drosophila which are ideal for genetic and biochemical studies. To capture accurately the intracellular biochemistry of transduction, complementary physiological, cytological, and biochemical approaches will b necessary. I propose to take advantage of genetic mutants of Drosophila using a physiological approach to study the roles of Ca2+ and cyclic-GMP an phosphoinositide pathways in excitation and adaptation. I will make use of a variety of existing visual mutants that appear to be faulty in these intracellular second messenger systems. Understanding how intracellular pathways interact may lead to a more basic understanding of the principles of cellular information processing.